MUTATION. 



145 



transmitted independently, it would have been presumed 

 that the difference between the albino and the black was 

 a difference produced by presence and absence of one sing- 

 le gene, and that the agoutis were the individuals hetero- 

 zygous for this gene, especially as they might be termed 

 intermediate. 



Those cases, where hybrids have been found to be intermed- 

 iate between two parents, and which look as if only one gene 

 made the difference between these parents, in as far as the 

 character studied was concerned, are few indeed. One of the 

 best studied cases is the almost classical instance of the Blue 

 Andalusian fowl. In some species of fowls three colours exist, 

 black, blue and white. In most of these, all the three colours 

 are bred and can be shown at poultry shows. This is the case 

 with black, blue and white Wyandottes, and Orpingtons, the 

 Dutch species of Witkuiven, sometimes called Polish, and the 

 Kraaikoppen. In some species however, as in the Andalusian, 

 and in one of the fighting breeds, only blues are wanted, and 

 shown. In most cases studied, we find that blues are produced 

 by crossing a black and a white, so long as the cross is confined 

 to differently coloured members of the same breed. Such blues, 

 if mated with each other produce about 50 % blues and the 

 remaining chicks will be blacks and whites, more or less splash- 

 ed. Blue bred to white gives as many blues as whites, and 

 blue to black produces as many blues as blacks. Both the 

 whites and the blacks breed true. This all works out well on the 

 hypothesis that the difference between black and white in 

 these breeds is caused by presence or absence of one single gene, 

 and we can either assume that this gene is a white-making 

 gene absent from black, or a black-making gene absent from 

 the whites, so long as we assume that heterozygotes, individu- 

 als having inherited it from only one parent show its action 

 only to a limited extent and are therefore only 50 % white, or 

 50 % black, which amounts to the same thing. Punnett in his 

 interpretation of breeding-experiments with Andalusian fowls 

 together with Bateson, choose the hypothesis that the blacks 



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